1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus, more particularly, to resolution conversion processing for restoring image data before pseudo high-resolution conversion processing with high precision based on image data converted into low-resolution and generated attribute data through the pseudo high-resolution conversion processing.
2. Description of the Related Art
Recent printers have gained remarkably high image quality, and had high-resolution engines, high-speed processors and large-capacity memories. However, since immense cost is required to prepare all these units, some methods have been proposed to grant both requirements of high image quality and high-speed, and lower cost.
For example, as a method which has been adopted in conventional electrophotographic printers, a method of exposing pixels on a photosensitive member so as to overlap each other at intermediate portion of their dot pitches using low-resolution image data, while the resolution of the engine maintains the low-resolution, is known. With this method, a latent image is formed so that an overlap portion of pixels also becomes an effective pixel. This method is called as a spot-multiplexing method in which an image is reproduced at a pseudo higher resolution than an actual resolution (for example, see JPA 4-336859).
However, in the method based on such spot-multiplexing, image forming is not stable and is difficult to be controlled when compared to the normal process. Also, the following problems are posed: the boundary between a background and object may become unclear in a character or line image, a character may be defaced, a jazzy may be generated, and so forth. In order to solve these problems, the following method has been proposed (for example, JPA 4-307270). In this method, for example, an received image of vector data is rendered to an image of raster data with a resolution lower than the resolution of the engine, the image of the raster data is converted in its resolution so as to finally match the resolution of the engine, and the converted image is output to the engine. As a result, a low-resolution is maintained in a controller, and an image is reproduced at a pseudo high resolution when it is output.
In a printer using the principle of JPA 4-336859, an image processor in the printer applies filter processing to convert a high-resolution image into a low-resolution image, and reproduces a pseudo high-resolution image from the low-resolution image using the spot-multiplexing method.
In this way, with the conventional technique using the spot-multiplexing method (for example, JPA 4-336859) and its improved technique, since a low-resolution image can be reproduced as a pseudo high-resolution image, for example, the memory size of an image storage area can be reduced. However, since a latent image is formed by overlapping two neighboring exposed portions so as to reproduce one dot, a formed image often becomes unstable and it is difficult to reproduce small fonts and thin lines and thus control the reproducing of the image.
In the above related art (for example, JPA 4-307270), after an image of vector data is rendered to an image of raster data with a resolution lower than the resolution of the engine and the raster data is stored, and then the resolution of the stored raster data is converted into the resolution of the engine when the stored raster data is output. For this reason, this related art can stably reproduce a high-resolution image while reducing the memory area in a controller without using any method that requires difficult control such as spot-multiplexing. However, since the rendered raster data has a low-resolution, image reproducibility is not enough when compared to a case in which the vector data is directly rendered to high-resolution raster data. Therefore, the above related art is a resolution conversion technique which is not sufficient to restore a high-resolution image.